Event Horizon Telescope Observations as Probes for Quantum Structure of Astrophysical Black Holes

TL;DR

This paper explores how quantum modifications near black hole horizons could cause observable, rapid changes in the black hole shadow, potentially detectable by the Event Horizon Telescope, offering insights into quantum gravity effects.

Contribution

It proposes that quantum fluctuations at the horizon scale can cause observable variability in black hole shadows, suggesting new observational tests for quantum gravity.

Findings

Quantum fluctuations can induce strong time dependence in black hole shadows.

Detection of shadow variability is feasible for large black holes like M87 with current EHT capabilities.

Rapid shadow variability may serve as a signature of quantum effects near horizons.

Abstract

The need for a consistent quantum evolution for black holes has led to proposals that their semiclassical description is modified not just near the singularity, but at horizon or larger scales. If such modifications extend beyond the horizon, they influence regions accessible to distant observeration. Natural candidates for these modifications behave like metric fluctuations, with characteristic length and time scales set by the horizon radius. We investigate the possibility of using the Event Horizon Telescope to observe these effects, if they have a strength sufficient to make quantum evolution consistent with unitarity. We find that such quantum fluctuations can introduce a strong time dependence for the shape and size of the shadow that a black hole casts on its surrounding emission. For the black hole in the center of the Milky Way, detecting the rapid time variability of its shadow will require non-imaging timing techniques. However, for the much larger black hole in the center of the M87 galaxy, a variable black-hole shadow, if present with these parameters, would be readily observable in the individual snapshots that will be obtained by the Event Horizon Telescope.